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it is the high range pH. It's from the high range pH bottle. I'll give you that it's not a perfect match for ANYTHING on that chart, but it's closest to about 7.6. If I test using the regular pH bottle it comes up blue every time.

You might also want to get a TDS monitor to see the difference in TDS from the warm tap water and your tank water.

I agree with this. I have been reading several articles lately stating it is differences in TDS and not pH that harms fish. pH in a natural water system like lakes and rivers often drop and raise more than a full point each 24 hours and the fish living in those bodies of water are fine because the GH and KH stay the same.

When I go fishing I just place a sharp rock in the water and sit there waiting for all the dead fish to float to the top... KingfisherBrutal honesty will be shown on this screen.I think my fish is adjusting well to the four gallon, He's laying on his side attempting to go to sleep on the bottom of the gravel.Tolerance is a great thing to have, so is the ability to shut up.I can explain it to you, but I can't understand it for you.

That would be what causes osmotic shock. It'd be the same thing as adding a bunch of rocks to your tank that released more minerals in the water, causing the water's hardness levels to rise (which can also cause fluctuations in pH - but that's usually a secondary symptom of change in hardness, being that whatever mineral is being released is either going to make your water more acidic or more basic).

Sorry to hear your are still having this problem. I can't properly remember what my feedback was the last time this issue was raised, but i seem to remember being suspicious about the water itself. i know that you can test the water using the bog standard stuff such as PH, KH etc.... Have you thought about getting samples professionally tested for other stuff? Such as taking a sample from the basement and a sample from the ground floor source and see what (if any) difference there is on paper. You never know and it might be useful to you anyhow.

Well, I have a working theory now after doing some extensive investigation of my water system. It's a combination of multiple factors.

1. My water heater doesn't work very well. It only puts out a little bit of hot water but soon can't do better than lukewarm. This is not a new problem but I think it's getting worse.
2. When I checked this tank with a more accurate thermometer, I found that it's sitting at about 81F. That temperature's fine for the loaches but it does mean supplying 35 gallons of water at that temperature is going to tax the water heater more than supplying 35 gallons of 75 degree water for my community tank.
3. The faucet in the basement has a very high flow rate - 11.5 gallons per minute. (presumably less with the python attached, I didn't measure that.) That means that while it fills the tank faster than other faucets, it also goes through hot water faster - requiring more vigilance when filling the tank.

So I believe what happened was this: During my last couple water changes, the temperature would be properly matched as I began filling the tank, but towards the end of the fill the water coming out of the python may have been as cold as 68F. By the time I was done the average temperature of the tank would not have changed more than a couple degrees, but in the mean time I would have created pockets of water up to 13 degrees colder than what the fish were used to... and those pockets of cold water would have settled to the bottom of the tank where the loaches live. Then, once the filters got moving the temperature around the fish would suddenly raise back up to nearly where it had been before.

Is that the sort of thing that would kill a delicate tropical fish like a clown loach but not harm a more hardy subtropical danio? I don't know for sure but it seems plausible. What do you guys think?

It sounds like a possible cause to me. We have already ruled out anything else I can think of.

Liters to Gallons conversion calculator
"Keeping fish for any period of time doesn't make you experienced if you're doing it wrong. What does, is acknowledging those mistakes and learning from them." ~Aeonflame
"your argument is invalid." ~Mommy1

I know I start to run out of hot water near the end of my waterchanges and will have to adjust the tapes I hook the python up to at least a few times to maintain the temp.

I use a digital thermometer and place the sensor right at the end of the python hose so I can always see what the temp of the replacement water is while I am refilling the tank. Maybe you could try something similar so you can make adjustments to the mix of hot & cool water and maintain a more stable temp while completing water change. Pic below

If you take your time to do the research FIRST, you can successfully set-up and keep ANY type of aquarium with ease."Not using a quarantine tank is like playing Russian roulette. Nobody wins the game, some people just get to play longer than others." - Anthony CalfoFishless CycleCycling with FishMarine Aquarium Info [URL="http://saltwater.aquaticcommunity.com/"]

I did another change on the tank last night. I did a few things differently: first, I had lowered the overall tank temperature to about 79, which meant I needed less hot water. Second, I turned it into a two-person job: I stayed at the tank the whole time with my hands in the water so I could feel how the temperature was coming out. My wife stayed at the sink (I used the upstairs kitchen sink and ran the hose down my basement steps) and adjusted the water temperature when I shouted to her from the basement. It worked (no fish deaths this time), but I constantly had to ask her to keep turning it up as time went on. And that was only a 35% water change instead of my usual 50%.

This Saturday I'm going to attempt to fix the water heater. I think I know what's wrong with it and in theory it should be a fairly simple fix... we'll see.

Well, after the first water change disaster I measured my nitrates at around 15, which means they were about 30 before the water change. I wouldn't want to go much higher than that. Keep in mind I've got dozens of danios and Der Poopinfuhrer the sailfin pleco in there too. With that said now that my loach bioload has been reduced by 62.5% I can probably get by with less.

Of course the ramifications for my future 300 gallon tank are significant, but we'll cross that bridge when we get to it.